Recently, the dye-sensitized solar cell (DSSC) has been attracting much attention as a potentially low-cost energy device. Typically, a DSSC consists of a dye-sensitized nanocrystalline semiconductor film on ITO (Indium-tin oxide) or FTO (fluorine-doped tin oxide) glass as the photo-anode, a platinized counter electrode which serves as the cathode, and an iodide/tri-iodide redox couple in proper mediator as the electrolyte. The working principle of a DSSC is summarized in five steps below, as shown in FIG. 1:    (1) Photo-excitation on dye molecules induces charge separation (see arrow 1 in FIG. 1).    (2) Charge (electron) injects into conduction band of mesoporous TiO2.    (3) Charge passes through outer circuit via electronic load (see arrow 2 in FIG. 1).    (4) Dye reduces to ground state by redox couple in the electrolyte (see arrow 3 in FIG. 1).    (5) Redox couple reduces on counter electrode by the charge coming from outer circuit (see arrow 4 in FIG. 1).
In a DSSC, the counter electrode functions as reduction reaction site such as:I3−+2e−→I−
This reduction reaction is vital since iodide ions are responsible for the regeneration of oxidized dye molecules. Once the dye regeneration can not catch up the dye oxidation (i.e. electron injection from dye molecule to CB of TiO2), whole conversion efficiency is obstructed and DSSC itself might deteriorate because iodine crystal may deposit on the counter electrode surface.
In the prior arts, the naked ITO or FTO glass shows extremely slow kinetics of tri-iodide reduction in organic solvents. In order to minimize the overpotential, catalyst material is deposited on ITO or FTO glass to speed up the reaction.
So far platinum (Pt) has been used almost exclusively as the catalyst material. However there are different methods to form a thin layer of Pt, the choice of which depends on the cost and efficiency.
Sputtering a thin layer of Pt on ITO or FTO is a method commonly used. This platinized electrode exhibits fair performance. However sputtering requires an ultra-high vacuum environment and is not suitable for mass production.
Papageorgiou et al. developed a method called “thermal cluster platinum catalyst” (Coord. Chem. Rev., 2004, 248, pp 1421). This method provides low Pt loading (about 2-10 μg/cm2), superior kinetic performance (charge-transfer resistance, RCT<0.1 Ωcm2), and mechanical stability with respect to conventional platinum deposition methods like sputtering, or electrochemical deposition.
Wang et al. (Surf. Interface Anal., 2004, 36, pp 1437) studied the stability of thermal cluster Pt (TCP) electrode by X-ray photoelectron spectroscopy; they found the electrochemical-catalytic performance of TCP might reduce slightly due to adsorbed iodide on TCP's surface and the electrochemical catalytic performance could be regenerated by reheating treatment. However, this method requires heating up to 380° C., which is not suitable for mass production.
Other materials such as carbon and conducting polymer were also proposed to be the catalyst for tri-iodide reaction in DSSC; these new materials usually require thicker porous films to be deposited on the substrate to obtain acceptable catalytic effect and are still being developed.